Depending on their purpose, material and condition, radioactive substances, especially those derived from the operation of nuclear reactors, when to be replaced and/or tested and transported and/or stored, are shielded from one another to prevent any further nuclear reactions engendered by their inevitably emitted neutrons. For ensuring the desired level of neutron absorption it has been customary to employ absorbers in the form of various storage shafts, canisters, tubes and similarly configured containers surrounding and thus shielding a neutron-emitting object. The use of such absorbers permits for instance the compact storage of neutron-emitting elements, especially fuel rods from nuclear power plants.
EP 0 385 187 A1 describes a fuel-rod storage rack where a number of absorber sheets form multiple shafts which enclose the fuel rods over their entire length. These absorbers are shafts or tubes which consist of a neutron-absorbing material, such as boron steel, i.e. an alloy steel with a boron concentration of 1 to 2%. Apart from the complexity of producing these absorbers, they are exceedingly cost-intensive, yet their effectiveness is limited due to the low boron content. In an attempt to increase the boron content, the deposition of a boron-nickel alloy was investigated. While the boron concentration can be increased up to 8%, the attendant cost increases by a factor of 10, ruling out any cost-effective use of this type of storage tubes.
For other purposes such as the transport and/or storage of radioactive materials, processes have been employed whereby layers of nickel are deposited on the metal surfaces of the containers.
U.S. Pat. No. 4,218,622 describes a composite absorber where a thin carrier foil or thin carrier sheet is coated with a polymer matrix in which boron carbide particles are embedded. The base material of the carrier foil or carrier sheet is preferably a fiberglass-reinforced polymer. The boron carbide particles are evenly distributed over the surface of the polymer matrix at a concentration of up to 0.1 g/cm2. When used in a fuel rod storage rack, this composite absorber is in the form of a foil or sheet up to 7 mm thick, suspended between an inner wall and an outer wall. U.S. Pat. No. 4,218,622 does not indicate whether a homogeneous distribution of the boron carbide particles over the surface of the polymer matrix can be assured in the long run, especially in view of possible surface abrasion.
EP 0 016 252 A1 describes a method for producing a neutron absorber. The process involves the plasma spraying of boron carbide, together with a metallic substance, onto a substrate, causing the boron carbide to be embedded in a matrix of the metallic substance. The process is also designed in a way that any oxidation of the boron is avoided. The absorber thus produced is intended to be chemically stable against a liquid medium such as that present in a fuel rod storage basin. The metal and boron-carbide layer applied by plasma spraying is at least 500 μm thick. The boron carbide content is about 50% by volume. Suitable metallic substances include aluminum, copper and stainless steel, with the substrate containing the same metallic substance as that in the sprayed-on layer. Obtaining sufficiently effective neutron absorption requires a relatively thick boron carbide-based layer. Specifically, the thickness of the layer is 3 to 6 mm.
The German provisional patent DE-AS 1.037.302 and German patent DE 2.361.363 describe a process whereby tubes and especially tin cans are electrolytically coated on their outer surfaces with an absorber material that protects them against radioactive radiation. Neither DE-AS 1.037.302 nor DE 2.361.363 provides any information on the procedural steps or equipment for the technical implementation of the change of the physiochemical state and material conversion involved in the application of the absorption material.
EP 0 055 679 A2 describes methods for producing shielding elements, whereby boron carbide is applied on the surface of the shielding element either by plasma coating or, following an electrolytic or chemical nickel preplating of the shielding element, by sprinkling boron carbide powder onto the surface, whereupon the shielding element is again nickel-plated by an electrolytic or chemical process. These methods allow only small amounts of boron carbide, on the order of magnitude of 20% by weight relative to the nickel content, to be applied on the surface. Consequently, very thick coatings are needed, so that these prior-art methods are not cost-effective. Nor have these methods really been employed in practice since from the process point of view they are not fully implementable. Sprinkling a powder on a surface is not a procedure that assures reliability in industrial production.
All of the prior-art methods and processes and the shielding elements produced by them can be considered uneconomical due to high production costs and material expenditures. Moreover, they limit variability in terms of the design of the shielding elements and any enhancement of their possible uses.
Producing boron steel is an extremely complex process. The steel is melted, the boron is enriched by complex methods to a valence of up to 10 and mixed with the molten steel. The result is boron steel containing boron at 1.1 to 1.4% by weight. This steel is very difficult to machine, it is extremely brittle and cannot be easily welded. Shielding elements produced from it are extremely heavy while offering only average absorption properties. As an example, storage container inserts, known as baskets, used for the interim storage of fuel rods, weigh as much as about 10 tons.
WO 98/59344 describes a method for producing a neutron-absorbing coating whereby the appropriate surfaces of a shielding element are provided with a boron-nickel layer, for which purpose the dispersion bath contains boron in its elemental form or as boron carbide. While it is possible to obtain a high rate of boron embedment, that rate is limited when boron is to be embedded in its elemental form, the layer is very hard and thus very brittle. Boron carbide offers only low conductivity, i.e. semiconductive characteristics at best, making it difficult if not impossible to control an electrolytic process. That in turn allows layers to build up only slowly and in poorly structured form. The relative movement involved results in a certain randomness in the structural pattern of the layer. That makes the process in general quite expensive since it is highly complex in terms of the materials used, process control and other parameters.